This is my first post in homebrew, so be gentle with me. I acquires an Ameritron 811 amp (not working) for a ridiculously small amount. After a simple repair, it worked well but the tubes were a bit soft. I ordered 572B tubes to replace the 811's originally supplied with the amp. When the new tubes arrived, I got to thinking (ALWAYS a dangerous thing) I couldn't help but notice that the 811 and the 811-H (800 watts with four 811 tubes) have the same power supply, so if were to increase output power, the power supply should be able to handle it. I've heard talk about mods to raise power and decided to give it a try.
I simply moved the "buck-boost" jumper to (a jumper inside the amp)110VAC.

PLATE VOLTAGE: This modification raised the measured idle plate voltage on my AL-811 from 1688 VDC to 1864 VDC. Which is well within the limits for 572B plate voltage(2700VDC). The measured filter caps voltage was 466 VDC (yeah, that's over the 450 volt rating). Plate voltage under load at 900W PEP into 50 ohms is
1600 VDC. While the idle plate voltage exceeds the rated 450 VDC of the stock capacitors, I have had no issues here....yet! There are other directly replaceable caps with higher voltage ratings such as 500VDC and 550VDC but I found them impossible to locate in stock. The stock caps do have a "surge" rating of 550VDC, but I guess I should replace them before a problem rears it's head.

PLATE CURRENT Plate current at 900W as measured by front panel meter is right at 750mA on voice peaks. Rated max plate current for 572B is 275mA x3 tubes or 825mA. Thus 750mA is within specs for this trio of tubes.

GRID CURRENT Grid current at 900W PEP on voice peaks is 120mA Max grid current for 3 572B tubes would be 150mA. Thus 120mA is within specs for these 572B's.

FILAMENT VOLTAGE This modification raised the 572B filament voltage from 6.3VAC to 6.9VAC. Close to the 6.6VAC max rating, but higher none the less. Do you think this higher filament voltage is something I should look into reducing, or are the extra 3 or 4, 10th of a volt negligible? And do you see any issues I'm missing here? I've had a few checks on the air and the amp seems to sound good, run clean, and give me more output than the original 300 watts. I haven't run it for any long periods of time.

I would definately be concerned with the higher filament voltage! On a lot of directly heated transmitting tubes, a decrease of the filament voltage by only 5% will show double the life of the tube. Running a tube at only 5% over voltage will shorten its live by up to 70%.
Curt

I have to agree with Carl here. Almost every time some one tries to hop up one of these amplifiers, they end up destroying the power supply, usually taking out the plate transformer. The power supplies were designed for just so much power and that was it.
Curt

You know, Amps are about the easiest things to build - especially if you already have the tubes - especially zero bias grounded grid triode amps. Power transformers to support 2kw are not that hard to come by and building a filter stack with computer grade caps is pretty simple too. As long as you aren't trying to make a sub-compact you could build yourself a nice amp for - not too much.

Arnie, Find me a transformer that will deliver 2300VDC at say 550mA so I can build me an amp. I have a lot of 4CX250 tubes in stock that I used in my 2 meter amp. They are all in very good condition. Most are NOS. A pair of them grid driven (using a 50 ohm dummy load on the input) will easily produce 600 watts. On my 2 meter amp, I ran them at a bit more plate voltage and a bit more output. I gave that amp away a number of years ago. I would like to build an HF amp but can never seem to locate a power transformer.
Jerry W5JH

I know a lot of hams used to use transformers that were built by Peter Dahl. Even the Gates BC-1T transmitter I kept on the air had a replacement Peter Dahl transformer in it as the original burnt up many years ago when the shack air conditioning failed during a spell of elevated temperatures in the summer.

Peter Dahl would wind one to your requirements if you had enough money to pay for it. However, I seem to recall someone saying the days of Peter Dahl transformers had come to a sad end. I never researched it, but maybe so.

Another place to look for a custom wound transformer may be Hammond. But custom wound transformers have always been way priced out of my league. I have aways been a firm believer in surplus transformers. Most of the time, they are very conservatively rated and you can draw a lot more power out of them without excessive heating than you can with commercial grade transformers.

Look for a plate transformer from an old BC-610 or the more modern T-368 or whatever it was. I picked up one about 40 years ago that was brand new and still in the wood box. It is about a cubic foot in size and has dual 120 volt primaries and the secondary is rated at 6336 volts center tapped at 500mA. It is an oil filled coffin type of transformer.

It came from Jim Jorgenson, K7RAJ in Orem, Utah back in the early 1970's when he had a bunch of them for sale and advertized them in the classified ads section of QST Magazine. Only cost me $40 for the transformer, but the motor freight company that hauled it got over $60! So it ended up being a 100 dollar transformer. My plans to use it with a bridge rectifier and four 872A rectifiers to power a 4-1000A amplifier never got off the ground after I went thru my divorce, so it still sits in the box.

It is yours for what I have in it, shipping it is something to considered.
Curt